(Rigging) Why are some of my vertex groups affecting blue areas?
Blue doesn't necessarily mean zero. When a vertex is only affected by one bone, a tiny amount of weight is enough to deform it like it had a weight of 1.0.
And even if a vertex has a main deforming bone near by (even with a weight 1.0) it can be noticeable when a bone with low influence is too far away and moves differently.

You can easily see the offending groups by going into Edit Mode, choosing a group of interest (Vertex Group list in the Vertex Group panel) and clicking on the Select button of the Vertex Groups panel. This will select all vertices in this group, no matter what weight they have. You can also remove the stray vertices from the group by selecting them only and clicking on the Remove button of the panel.



You can also see the groups of a vertex in the N-Panel and remove them there (click the X button of the group you want to remove). When you choose Deform, only the deforming groups are visible.



In Weight Paint mode you can also use the Use Zero Weights option (T-Panel,Options) to show problems in your weighting. Select Active and all vertices with zero weight of the selected bone are shown in black. As you can see, selecting the leg shows weights in blue around the head. Which means the leg bone is deforming the vertices of the head.



You can also see problems with no weights at any bone using All. As you can see there are some black areas near the leg. This would mean these vertices will not move at all. Note that this also includes non-deforming groups, so seeing no black vertices can be misleading.



To clean vertex group weights (in Weight Paint mode) use the Clean tool (T-Panel,Weight Tools). Click the Clean button and the tool options will be shown as in the image below. You can select a threshold in the Limit field. All vertices below this weight will be removed from the group automatically:



Another thing useful while weight-painting is to use Wireframe Display mode (with Limit selection to visible turned on) frequently. That way the shading doesn't obscure the weight colors. It also helps to locate the vertices better so you don't have to guess where to aim with your cursor.



Usually these problems stem from using a blend mode like Mix while painting. If you want to remove weights completely, it is often better to use Subtract with full Strength and Weight.